Rust Error E0757: ffi_const and ffi_pure Attributes Conflict

Understanding Rust Error E0757

Rust Error E0757 occurs when a function has both the ffi_const and ffi_pure attributes. These attributes are used to define the behavior of external functions when interfacing with C code. However, they cannot be used together, as the ffi_const attribute provides stronger guarantees than ffi_pure.

Here's an example of erroneous code that triggers Rust Error E0757:

#![allow(unused)]
#![feature(ffi_const, ffi_pure)]

fn main() {
    extern "C" {
        #[ffi_const]
        #[ffi_pure] // error: `#[ffi_const]` function cannot be `#[ffi_pure]`
        pub fn square(num: i32) -> i32;
    }
}

Solution to Rust Error E0757

To resolve Rust Error E0757, remove the ffi_pure attribute since ffi_const already provides stronger guarantees. Here's the corrected code:

#![allow(unused)]
#![feature(ffi_const)]

fn main() {
    extern "C" {
        #[ffi_const]
        pub fn square(num: i32) -> i32;
    }
}

Understanding ffi_const and ffi_pure Attributes

ffi_const represents a function with no side effects and whose return value depends only on the values of its arguments, meaning that the function will always return the same output for the same input. It allows the compiler to perform optimizations like common subexpression elimination or constant folding.

ffi_pure, on the other hand, represents a function with no side effects but is more lenient than ffi_const. Its return value may depend on global memory, and the function may not always return the same output for the same input.

For a more detailed explanation, consult the GCC documentation on Common Function Attributes. The unstable Rust Book also covers ffi_const and ffi_pure.